Some mountainous areas of Puerto Rico are going to be without electricity for as long as eight months, the United States Army Corps of Engineers said this week. It has been three months since Hurricane Maria swept through the island, knocking down tens of thousands of power poles and turning off virtually everyone’s lights. The amount of power being generated there is at about 65 percent of capacity — and it has been stuck around that level since late November.

We went to the Army Corps and the Puerto Rico Electric Power Authority to find out why.

How long is it going to take to get the lights on?

Most of the island will have power by the end of February, said Gen. Diana M. Holland, commander of the South Atlantic Division of the Army Corps of Engineers. But the last stretch, the hard-to-reach rural areas, will not get power until the end of May, just in time for the 2018 hurricane season.

“Our power grid has never seen anything like this,” said Justo González, the interim director of the power authority, known as Prepa.

The Army Corps said the areas that were expected to take the longest were the central towns of Lares, Utuado and Adjuntas — together home to about 80,000 people.

What’s taking so long?

“The sheer amount of work,” said José E. Sánchez, an engineer at the corps who leads the power restoration task force. “The first time I saw it, I thought: ‘This is going to take a long time.’”

The damage to an already outdated and poorly maintained grid was comprehensive. Lines went down, poles snapped, towers fell and substations flooded. There are 30,000 miles of electrical line in Puerto Rico, and about 63 percent of it was affected.

To underscore the scope of the work: Almost 50,000 power poles need to be repaired or replaced. Add 500 towers to that. And the towers are so heavy that helicopters cannot carry them, so they have to be installed in stages. It can take up to 10 days just to finish one.

And some of the supplies, such as the 30,000 power poles that were ordered on Oct. 6 — 16 days after the storm — are beginning to arrive only now. Some 400 miles of cable are expected to reach the island in the next two weeks, Mr. González said.

Why is there a delay in sending supplies?

Many of the items simply take a long time to manufacture, ship and offload, General Holland explained.

“The thing that challenges every mission that we’re doing here has been the logistics, the materials, just the physics of getting here,” she said.

Puerto Rico has struggled to obtain enough transformers, electrical fittings and electrical insulators, according to Prepa — to the point that crews have been assigned to recycle existing materials while they wait for new ones to arrive. That means uninstalling equipment from one place, certifying that it works and installing it somewhere else, without taking power away from the first location to light up the second. That process is inefficient, according to Mr. González.

How many people have power and how many don’t?

Prepa would not venture a guess.

The apparatus that allows the agency to know which customers have power, known as the outage management system, has been giving readings that are so out of whack nobody trusts them to be true, Mr. Sánchez said. So the government instead has been reporting how much power is being generated.

That number has fluctuated around 65 percent for weeks now. But because critical areas like hospitals and water treatment plants that consume lots of power were energized first, this does not mean 65 percent of households have power.

The generation amount has stalled, Mr. González said, because people are using less power as temperatures cool down and businesses and schools close for the holidays.

On Friday, Mr. González said that 73 of the island’s 78 municipalities have some sort of power, even if it comes from generators installed by the Army Corps of Engineers to create temporary power micro-grids. (Mr. Sánchez put the number of cities with electricity at about 55.)

The lights are mostly back on in Ponce. Culebra is running entirely on a generator the Army Corps installed at the local plant. The five municipalities that remain completely dark are Ciales and Morovis, in mountainous central Puerto Rico, and Maunabo, Naguabo and Yabucoa, in the southeast, where Hurricane Maria’s eye made landfall. The Army Corps is planning to put generators there to light at least the town squares of those cities.

Who is responsible for restoring power and how much is this work costing?

Power restoration is a joint responsibility of Prepa and the Army Corps, Mr. González said. Prepa establishes priorities for which parts of the grid to tackle first. The Army Corps is charged with buying the materials.

Some 3,500 people — from Prepa, the Army Corps and private contractors — are working on restoration, with about 1,000 more expected to arrive in mid-January from mainland utilities that signed mutual-aid agreements with Puerto Rico, Mr. González said.

The Federal Emergency Management Agency has allocated $1.8 billion to the Army Corps.

Prepa has also spent about $75 million so far: About $40 million went to an Oklahoma company, Cobra, that Prepa hired to do repair work. More than $35 million went to Whitefish Energy Holdings, the Montana company Prepa hired as part of a widely criticized $300 million contract. FEMA has said it would not reimburse Puerto Rico for the Whitefish contract, but Mr. González said they were hoping the agency would reconsider.

How will the electricity be delivered?

Mr. González said his goal was to restore power by whatever means possible, even if it is with temporary generators or battery systems. Some homes were left in such bad shape that they might not be able to connect to the grid.

“What we want to do is bring electricity,” he said, no matter what form it takes. “Power is power.”